Method for isolating epsilon-caprolactam material

ABSTRACT

Epsilon -CAPROLACTAM MATERIAL SUCH AS A MONOMER, OLIGOMERS AND POLYMERS OF Epsilon -CAPROLACTAM AND MIXTURES OF THESE COMPOUNDS ARE PURIFIED FROM IMPURITY COMPOUNDS SUCH AS PHOSPHORIC ACID AND IONIZED METAL COMPOUNDS CONTAINED THEREIN BY BRINGING THE Epsilon -CAPROLACTAM MATERIAL INTO CONTACT WITH AN AQUEOUS SOLUTION CONTAINING 15 TO 75 PERCENT BY WEIGHT OF AT LEAST ONE ALKALI METAL PHOSPHATE TO EXTRACT THE IMPURITY COMPOUND FROM THE Epsilon -CAPROLACTAM MATERIAL INTO THE PHOSPHATE AQUEOUS SOLUTION AND, THEREAFTER, SEPARATING THE Epsilon -CAPROLACTAM MATERIAL FROM THE PHOSPHATE AQUEOUS SOLUTION BY WAY OF SETTLING OR CENTRIFUGING THE MIXTURE.

United States Patent 11 1 Izawa et al.

METHOD FOR ISOLATING EPSILON-CAPROLACTAM MATERIAL Inventors: NobuoIzawa; Toshihiko Kohno,

, both of Sakai, Japan Assignee: Kanebo, Limited, Tokyo, Japan Filed:Feb. 11, 1974 Appl. No.: 441,061

us. (:1 260/239.3 A; 260/78 sc; 431/7 1m. (:1. c0712 201 12; 0071)201/16;

' 0080 64/14 Field 61 Search 260/2393 A, 78 s, 78 SC References CitedUNITED STATES PATENTS 3/1960 Weise 260/2393 A Primary ExaminerHenry R.Jiles Assistant Examiner-Robert T. Bond Attorney, Agent, or FirmPaul &Paul ABSTRACT e-caprolactam material such as a monomer, oligomers andpolymers of s-caprolactam and mixtures of these compounds are purifiedfrom impurity compounds such as phosphoric acid and ionized metalcompounds contained therein by bringing the e-caprolactam material intocontact with an aqueous solution containing 15 to 75 percent by weightof at least one alkali metal phosphate to extract the impurity compoundfrom the e-caprolactam material into the phosphate aqueous solution and,thereafter, separating the e-caprolactam material from the phosphateaqueous solution by way of settling or centrifuging the mixture.

17 Claims, No Drawings METHOD FOR ISOLATING EPSlLON-CAPROLACTAM MATERIALThe present invention relates to a method for purifying an e-caprolactammaterial, more particularly, relates to a method for purifyinge-caprolactam compound from phosphoric acid, ionized metal compounds andmixtures of two or more of the above-mentioned compounds.

The term e-caprolactam material used herein refers to any e-caprolactammonomer, oligomers and polymers and mixtures of two or more of thesecomcaprolactam material to be depolymerized contains metal ions, themetal ions cause deactivation of the phosphoric acid catalyst anddeposition of a large quantity of metal salts in the depolymerizationvessel. These behaviors result in interruption of the depolymerizationreaction, decrease in the yield of the ecaprolactam monomer anddeterioration in quality of the e-caprolactam monomer.

Generally, a crude e-caprolactam monomer which has been prepared by aBeckmann rearrangement reaction from cyclohexanone oxime, ordepolymerization of an e-caprolactam raw material, polymer or oligomer,at a high temperature, is purified by distillation in the presence of asmall amount of an alkali metal hydroxide or carbonate. Accordingly, thedistillation residue from the above process contains concentrated alkalimetal compound and, therefore, scrapping of the concentrated metalcompounds accompanies scrapping of a large amount of the e-caprolactamraw material. This results in a reduction in the yield of the purifiedecaprolactam monomer. Also, the scrapping of the large amount of thedistillation residue by way of dumping, reclaiming or discharging withwater, may result in environmental pollution. On the other hand, inburning the distillation residue, the concentrated metal compoundsrestrict the smooth combustion of the distillation residue. Further, themetal compounds corrode the combustion furnace for the distillationresidue. Sometimes the destruction by fire of the distillation residuein the combustion furnace becomes practically impossible due to theinfluence of the concentrated metal compounds.

In the case where the e-caprolactam waste material, oligomer or polymer,is depolymerized in the presence of the phosphoric acid catalyst whiledistilling out the resultant e-caprolactam monomer the reaction residuefrom the above process contains concentrated phosphoric acid, a brownresinous substance which is a destruction product of the e-caprolactamwaste material, and a concentrated organic or inorganic substance whichhas been initially contained in the e-caprolactam waste material. Inorder to smoothly burn up the reaction residue, it is necessary toremove the phosphoric acid and the metal phosphates which has beenproduced by reaction of phosphoric acid as the depolymerization catalystwith the metal compoundsoriginally contained in the e-caprolactam wastematerial. This is because the phosphoric acid and metal phosphates notonly damage the combustion furnace but greatly obstruct the combustionof organic material. Also, sometimes, the reaction residue contains alarge quantity of metal phosphates, especially, alkali metal phosphates,whereas the destruction product is in a small amount. In this case, suchmetal phosphates make it very difficult to proceed with thedepolymerization.

This is because a large amount of the metal phosphates which areinsoluble eitherin'melted e-caprolactam polymer or e-caprolactamoligomer and, therefore, dispersed in the form of fine solid particlesin the depolymerization system, causes an increase in the apparentviscosity of the depolymerization system. That is, the low fluidity ofthe depolymerization system results in difficulty in attaining a uniformreaction between the e-caprolactam waste material, polymer or oligomer,and phosphoric acid and in unsmoothness in distilling out the resultinge-caprolactam monomer. This further results in a low quality and yieldof e-caprolactam monomer being produced in the depolymerization system.

Because of the above circumstances, a method has been desired whichwould allow removing metal ions, especially, alakli metal ions,phosphoric acid and metal phosphates from an e-caprolactam monomer,oligomer or polymer, at high efficiency.

Such a method should be capable of being applied to the e-caprolactammaterial to be depolymerized, distilled or burnt up, or the reactionresidue derived from a reaction process for recovering -caprolactammonomer, in order to smooth the depolymerization, distillation anddestruction by fire of the e-caprolactam material. However, no suchmethod has previously been known.

The object of the present invention is to provide a method for purifyingan e-caprolactam compound from metal compounds and phosphoric acid insimple operation and at a high efficiency.

The other object of the present invention is to provide a method forpurifying an e-caprolactam compound from metal compounds and phosphoricacid, the isolated e-caprolactam compound being suitable fordepolymerization, distillation and combustion without difficulty.

The above objects can be accomplished by the method of the presentinvention comprising the steps of bringing an e-caprolactam materialselected from the group consisting of a monomer, oligomers and poly mersof e-caprolactam and mixtures of two or more of the above-mentionedcompounds and containing at least one compound selected from the groupconsisting of phosphoric acid and ionized metal compounds, into contactwith an aqueous solution containing 15 to percent by weight of at leastone alkali metal phosphate to extract said phosphoric acid and ionizedmetal compound from the e-caprolactam material into the alkali metalphosphate aqueous solution, and separating said e-caprolactam materialphase from the alkali metal phosphate aqueous'solution phase.

The term metal compound, used herein refers to metal salts, metalhydroxide and other metal compounds existing in the e-caprolactammaterial.

The term alkali metal phosphates used herein in cludes mono-alkali metaldihydrogen phosphates, di-

3 alkali metal monohydrogen phosphates, tri-alkali metal phosphates andmixtures of two or more of the abovementioned phosphates.

The e-caprolactam material to which the method of the present inventionis applicable, may be selected from the group consisting of a. reactionresidue containg an e-caprolactam oligomer, monomer or a mixture thereofand alkali metal compounds, which has been obtained from the processwherein the e-caprolactam polymer is washed with water, and the wastewater-containing e-caprolactam monomer, oligomer or mixture thereof isdistilled in the presence of an alkali metal, hydroxide, carbonate, orhydride of alkali metal or mixture of two or more of the foregoingsubstances,

b. distillation residues containing an e-caprolactam monomer and alkalimetal compounds, which has been obtained from a process wherein ane-caprolactam monomer is polymerized in the presence of water as acatalyst, and a portion of the e-caprolactam monomer is evaporatedtogether with water and the e-caprolactam monomer is distilled in thepresence of an alkaline alkali metal compound.

c. reaction residues produced in processes for depolymerizing ane-caprolactam polymer, oligomer or mixture of the polymer and oligomerin the presence of phosphoric acid as a catalyst,

d. reaction residues containing alkali metal com pounds produced in adistillation process for refining a crude e-caprolactam monomer whichhas been produced by depolymerizing an e-caprolactam polymer or oligomeror a mixture of the polymer and oligomer in the presence of phosphoricacid as a catalyst,

e. distillation residues produced in processes wherein a crudee-caprolactam monomer is produced by the Beckman rearrangement reactionof cyclohexanone oxime and the resultant crude e-caprolactam isdistilled in the presence of an alkaline alkali metal compound,

f. reaction residues produced in processes for depolymerizing ane-caprolactam polymer or oligomer or a mixture of the polymer andoligomer in the presence of an alkali metal compound as catalyst,

g. an e-caprolactam polymer, oligomer or mixtures of the polymer andoligomer which has been prepared by polymerizing an e-caprolactammonomer in the presence of a catalyst consisting of at least onesubstance selected from the group consisting of alkali metals, alkalimetal hydroxides, alkali metal carbonates and alkali metal hydrides;

h. mixtures of two or more of the e-caprolactam materials as statedabove.

The above-mentioned distillation residues (a), (b), (d), and (e)sometimes, contain manganese compounds derived from potassiumpermanganate, with which the e-caprolactam monomer has been refinedbefore the distillation.

The above-mentioned e-caprolactam materials may be admixed with eachother.

The e-caprolactam material to which the method of the present inventionis applicable, generally contains 0.1 to percent by weight of alkalimetal ions calculated in terms of an alkali metal hydroxide. However,the method of the present invention is applicable and has highefficiency even for the e-caprolactam material containing about 0.01percent by weight of metal ions. especially, alkali metal ions,calculated in term of a metal hydroxide.

In the case where an e-caprolactam polymer or oligo mer is depolymerizedand the resultant e-caprolactam monomer is recovered by way ofdistillation, the depolymerization reaction residue, to which the methodof the present invention is applicable, contains phosphoric acid whichhas been utilized as a depolymerization catalyst, netal phosphates whichhave been produced by reaction of the phosphoric acid with metal ion,especially, alkali metal ion, originally contained in the e-caprolactampolymer or oligomer, or which have been contained in the e-caprolactampolymer or oligomer as a component of polymerization catalyst, ormixtures of the phosphoric acid and the metal phosphates. The contentsof the impurities, of course, depends on the amount of the phosphoricacid used in the depolymerization, the amount of the metal ionsinitially contained in the e-caprolactam polymer and type of thedepolymerization process, that is, continuous or batchwisedepolymerizations. In the case where batchwise depolymerizations arerepeated, sometimes the final reaction residue contains about 80 percentphosphoric acid. Compared with this, in the case of continuousdepolymerization, sometimes the reaction residue contains about 1percent phosphoric acid. The method of the present invention can beapplied to either the reaction residue containing a very large amount ofphosphoric acid or the reaction residue containing a very small amountof phosphoric acid.

The e-caprolactam material usable for the method of the presentinvention may contain impurities other than the phosphoric acid andalkali metal compound, for example, titanium dioxide pigment, carbonblack manganese pyrophosphate used as a light proofing agent,anti-flaming agent containing copper salt or bromine compound, highboiling point compounds such as oiling agents, and basic compounds,resinous substances and carbonized substance which have been producedduring high temperature polymerization or depolymerization. The totalcontent of these impurities in the e-caprolactam material to be treatedby the method of the present invention depends on the history of thematerial, for example, type and quality of the ecaprolactam polymer oroligomer used in depolymerization and type of the depolymerization anddistillation, and depolymerization conditions. Generally, this totalcontent is about 20 percent or less.

The alkali metal phosphate aqueous solution usable for the method of thepresent invention contains 15 to 75 percent, preferably, 20 to percent,more preferably, 30 to 60 percent by weight of the alkali metalphosphates. If the concentration of the alkali metal phosphate aqueoussolution is lower than 15 percent by weight, the e-caprolactam monomer,e-aminocaproic acid which is a ring-cleaven hydrated product of anecaprolactam monomer, and water soluble organic impurities undesirablytend to move from the e-caprolactam material phase into the phosphateaqueous solution phase and the phosphate aqueous solution phase has alow separating property from the e-caprolactam material phase. Further,in this case, the metal ions and phosphoric acid are incompletelyextracted from the e-caprolactam material phase into the phosphateaqueous solution. If the phosphate aqueous solution has a concentrationhigher than percent by weight, the separating property of the phosphateaqueous solution phase from the e-caprolactam material phase isunsatisfactory. This results in insufficient extraction of the metalions and phosphoric acid,

The process of the present invention is basically premised on thegrounds'that the e-caprolactam monomer, oligomer and polymer areincompatible with the concentrated alkali metal phosphate aqueoussolution and that the ionized metal compounds, especially, the ionizedalkali metal compounds and phosphoric acid are distributed in the alkalimetal phosphate aqueous solution at a very higher distributioncoefficient than in the e-caprolactam monomer, oligomer and polymer.

The alkali metal phosphates usable for the method of the presentinvention may be lithium, sodium or potassium phosphates or mixtures oftwo or more of the above-mentioned phosphates.

The phosphate aqueous solution usable for the method of the presentinvention may be prepared by dissolving, in water, the mono-alkali metaldihydrogen phosphates, di-alkali metal monohydrogen phosphates,tri-alkali metal phosphates, alkali metal polyphosphate mixtures of theabove phosphates, or mixtures of phosphoric acid'or polyphosphoric acidsand alkali metal hydroxides, alkali metal carbonates. This phosphateaqueous solution may be in a pH range from 4.0 to 12.5. The tri-alkalimetal phosphate aqueous solution is more highly alkaline and, therefore,suitable for extracting phosphoric acid and acid salts rather thanalkaline salts from the e-caprolactam material.

Sometimes the phosphate aqueous solution contains a single phosphatebut, sometimes, it contains a mixture of two types or more ofphosphates. In a preferable aspect, mixtures of a monoalkali metaldihydrogen phosphate and a di-alkali metal monohydrogen phosphate aredissolved in water at a pH from 5 to 9. This aqueous solution isapproximately neutral and, therefore, has no tendency to give alkalinesubstance or acid substances to the e-caprolactam material. Further,this aqueous solution has very little tendency to give neutral salts tothe e-caprolactam material. If an acid or alkaline substance is movedfrom the phosphate aqueous solution phase into the e-caprolactammaterial phase, the isolated e-caprolactam material has difficulty incombustion distillation and depolymerization. The acid or alkalinesubstance causes damage to the combustion furnace and obstruction of thecombustion of the ecaprolactam material.

The contact of the e-caprolactam material with the phosphate aqueoussolution-is carried out at a temperature of, preferably, 60 to 300C,more preferably, 100 to 250C. Even if the contact is effected at atempera ture above 300C, the extraction effect is not enhanced. Further,such high temperature extraction undesirably requires an apparatusresistant to very high pressure. If the extraction temperature is lowerthan 60C, the e-caprolactam material is in solid phase. This isdisadvantageous for completing the extraction within a short time.Accordingly, it is more preferable to effect the extraction at atemperature of 100 to 250C under a small positive pressure of l to 40kg/cm The ratio of the weight of the e-caprolactam material to theweight of the phosphate aqueous solution is preferably in a range from 1to 2 l. The mixture of the e-caprolactam material and the phosphateaqueous solution is preferably stirred from a few minutes to tenminutes. The stirring time is adjustable in response to the form of thee-caprolactam material, extraction temperature, type of apparatus forthe extraction and type of stirrer. Generally, the stirring time isshorter than 2 hours. The extraction may be carried out in any typeoperation, batchwise or continuous, and in any conventional apparatus.For example, a settler with a stirrer is usable for the extraction. Aconventional centrifugal separator is useful for completely purifyingthe ecaprolactam material phase from the phosphate aqueous solutionphase. I

The phosphate aqueous solution may be repeatedly or continuously used toextract the phosphoric acid and metal compounds. In this case, theconcentration of the phosphate aqueous solution varies with therepeating number of the extraction. Accordingly, if desired, theconcentration is adjusted by adding water, phosphoric acid, alkali metalhydroxide, alkali metal carbonate or alkali metal phosphate into theextraction system.

If it is necessary, before the extraction operation, any of theabove-mentioned compounds and water may be added in an amount calculatedfrom the concentrations and compositions of the used alkali metalphosphate aqueous solution which will be extracted after the extractionoperation is completed, into the extraction system.

In the event that some water-insoluble and chemically stable substance,such as titanium dioxide pigment and carbon black, in the e-caprolactammaterial, is moved into the phosphate aqueous solution during theextraction, the substance can be removed by a suitable operation, forexample, filtration and decantation.

The alkali metal phsophate extracted into the phosphate aqueous solutionmay be recovered from the aqueous solution to utilize in various fields.For example, the recovered phosphates can be converted to sodiumtripolyphosphate usable as a detergent builder or to tetrasodiumpyrophosphate usable for cleaning waste water or as a boiler compound.

The purified e-caprolactam material contains a small amount of water anda negligible amount of metal ions, phosphoric acid or both the abovesubstances. However, these small amount of impurities do not interferewith depolymerization, distillation and combustion of the e-caprolactammaterial.

In order to depolymerize the purified e-caprolactam material free frommetal ion, a suitable amount of phosphoric acid or a reaction residuecontaining phosphoric acid is mixed intothe purified e-caprolactammaterial, and the mixture is heated at a temperature of 200 to 320Cunder an absolute pressure of 0.1 to 3 atmosphere. The phosphoric acidis preferably in a concentration of 0.5 to 30 percent by weight. Aportion (not larger than 20 percent by mole) of the phosphoric acid tobe added to the depolymerization system may be substituted by monosodiumdihydrogen phosphate. The s-caprolactam material to be depolymerized maybe fed into a depolymerization reactor batchwise or continuously. If itis necessary, the e-caprolactam material may be preheated to removewater therein, and, thereafter, heated to the desired depolymerizationtemperature. The depolymerization reaction may be promoted by blowingsuperheated steam into the depolymerization system. The depolymerizationmay be carried out without blowing of the superheated steam under areduced pressure of l to 50 mmHg. However, in this case, the resultant e-caprolactam monomer has a relatively low quality.

The depolymerization of the e-caprolactam material purified inaccordance with the method of the present invention can be very smoothlycarried out as it contains no or a negligible amount of metal compoundand, therefore, the reaction mixture has a high fluidity. After thedepolymerization is completed, it is observed that no or a negligibleamount of adhesive substance stains the inside wall surfaces of reactoror pipes. Therefore, the depolymerization can be continued over a verylong time period with a high efficiency and the resultant e-caprolactamis of an excellent quality.

The e-caprolactam monomer purified by the method of the presentinvention may be subjected to distillation in the conventional methods.If it is necessary, the purified e-caprolactam monomer is furtherpurified with activated carbon, ion exchange resin, permanganates,bichromates or alkali compounds, before the distilla tion. The purifiede-caprolactam monomer may be distillated alone or mixed with a freshe-caprolactam monomer and then, distilled together. The distillation maybe effected batchwise or continuously at a temperature of 120 to 200Cunder a pressure of to mmI-Ig, using a conventional distillator such asmultistage distillation tower having perforated plates which is of lowpressure loss, thin film type distillator or single distillatorfThee-caprolactam monomer may be preheated before the distillation to removelow boiling point substances such as water.

In the case where the purified e-caprolactam material contains anoticeable amount of lowly volatile substances such as e-aminocaproicacid and e-caprolactam oligomer, it is advantageous that thee-caprolactam material be mixed with an e-caprolactam polymer material,such as waste poly-e-caprolactam, to be depolymerized, Thedepolymerization is, of course, carried out by the conventional methodfor the e-caprolactam polymer or oligomer.

If the isolated e-caprolactam material derived from the reaction residueof the depolymerization reaction process merely contains no or anegligible amount of impurities other than phosphoric acid and alkalimetal phosphate, it is desirable to recycle the purified ecaprolactammaterial to the depolymerization reactor. This is effective forenhancing the yield of the ecaprolactam monomer.

For the purpose of burning up the purified ecaprolactam material, it issent to a combustion furnace in the melt state or after solidifying. Inorder to prevent imperfect combustion and generation of nitrogen oxides,the combustion is carried out generally at a temperature of 500 to 900C.As the phosphoric acid and alkali metal compounds which tends to damagethe combustion furnace and obstruct the combustion of organic substancesis removed, all of conventional combustion furnaces for conventionalsynthetic plastics can be utilized for burning up the isolatede-caprolactam material. The combustion furnace may be selected fromfixed combustion bed type, converter type, spray combustion type, vortexcombustion type and fluidized combustion bed type furnaces. The purifiede-caprolactam material may be mixed with a combustible material andburnt up together therewith.

In a preferable embodiment of the method of the present invention, analkali metal phosphate aqueous solution is brought into contact with ane-caprolactam polymer or oligomer containing alkalis or alkaline saltsto be sent to a depolymerization process, or an ecaprolactam monomercontaining alkali metal ions to be recycled to a distillation process ordepolymerization process or to be sent to destruction by fire,'to-removethe alkalis and alkaline salts and, then, brought once again intocontact with s-caprolactam material containing phosphoric acid or acidsalts of alkali metals to eliminate the phosphoric acid and the acidsalts. Of

8 course, the above operation may be carried out in the reverse order tothe above. That is, an alkali metal phosphate aqueous solution isutilized twice or more for separately extracting alkali and alkalinesalts and phosphoric acid and acid salts in alternate operations.

The ecaprolactam material containing alkaline salts or alkalis may bepreliminarily blended with the ecaprolactam material containingphosphoric acid or acid salts. In this case, the extraction is carriedout in one operation. This is effective not only for minimizingconsumption of the phosphoric acid and alkali metal phosphates but foreconomizing equipment cost and operational cost.

As detailed above, the method of the present invention is effective forremoving metal compounds, particularly, alkali metal compounds, andphosphoric acid from the e-caprolactam material by simple and easyoperations. This removal is effective for smoothing thedepolymerization, distillation and combustion of the e-caprolactammaterial and enhancing the quality and yield of the recoverde-caprolactam monomer. Further, the method of the present invention canbe carried out either, batchwise or continuously to recover and utilizethe waste e-caprolactam material without loss in quan tity of thee-caprolactam material and creation of environmental pollution.

The following examples are intended to illustrate the method of thepresent invention but are not intended to limited the scope thereof. Inthese examples, parts and percentages are by weight unless otherwiseindicated, and the contents of ionized metal compounds and phosphoricacid in the e-caprolactam material were de termined by the methodwherein the e-caprolactam material was ashed and then, the ash wasdissolved in hot water, a diluted hydrochloric acid aqueous solution ora diluted sodium hydroxide aqueous solution and the solution wastitrated, or the ash was dissolved in nitric acid and the solution wassubjected to an atomic light absorption analysis. Or, some alkali metalions were titrated in such a manner that the sample to be analized wasdirectly dissolved or'suspended in water and the aquous solution orsuspension was titrated with a 0.1 N hydrochloric acid aqueous solution.

The term PM value used in the examples is an index of the quality of thepurified e-caprolactam monomer, and was determined in such a manner that1 g of e-caprolactam was dissolved in 100 ml of distilled water, intothe solution was mixed 1 ml of an aqueous solution containing 0.01 moleof potassium permanganate, and the time in seconds required to changethe color of the above mixture solution to a standard solution (which isan aqueous solution of 3 g of CoCl .6H O and 2 g of CuSO .5H O in 1litre of water) was measured.

The acid value" of the e-caprolactam monomer used in the examples wasdetermined in such a manner l that 5 g of e-caprolactam monomer weredissolved in 50 ml of water, the aqueous solution was titrated withabout ten times the minimum fluidization velocity. During thecombustion, the combustion furnace was maintained at a temperature ofabout 800C by'spraying water thereinto and adjusting the temperature ofthe air blown thereinto.

The combustion test was continued for 48 hours unless difficulty wasobserved, to burn up the e-caprolactam material in a weight of abouttwice the weight of the bed material.

EXAMPLE 1 A poly-e-caprolactam was prepared by polymerizings-caprolactam, and a mixture of non-reacted ecaprolactam ande-caprolactam oligomer were recovered by way of extraction. Further, thenon-reacted ecaprolactam monomer was recovered from the mixture by wayof distillation. kg of the distillation residue which contained 60percent of e-caprolactam oligomer and 14 percent of alkali in term ofNaOH, was heated to a temperature to prepare a homogenous solution andthen, uniformly mixed with 6 kg of an aqueous solution of 90C containing42 percent of monosodium dihydrogen phosphate (NaH PO and having a pH of4.0 by stirring for 2 minutes using a stirrer. The mixture was settledwithin a separating funnel heated at a temperature of 909C for minutes,and the e-caprolactam oligomer phase was separated from the phosphateaqueous solution phase.

As a result of chemical analysis, the e-caprolactam oligomer phasecontained no free alkali, 0.3 percent of sodium dihydrogen phosphate and0.2 percent of phosphoric acid.

COMPARISON EXAMPLE 1 5 kg of the same residue from the e-caprolactammonomer distillation as in Example 1 was mixed with 10 kg of an aqueoussolution of 90C containing 23 percent of sodium sulfate and the mixturewas vigorously shaken. The mixture was settled in a warm separatingfunnel to separate the e-caprolactam oligomer phase from the sodiumsulfate aqueous solution phase. As a result of chemical analysis, thee-caprolactam oligomer phase contained 0.15 percent of free alkali. Forfurther comparison, the same procedures as in Comparison Example 1 wererepeated using water instead of the sodium sulfate aqueous solution. Thee-caprolactam oligomer was uniformly dissolved in water. Therefore, itfailed to separate the e-caprolactam oligomer phase from the waterphase.

For the other comparison, the same procedures as in Comparison Example 1were repeated using an aqueous solution containing 10 percent ofphosphoric acid instead of the sodium sulfate aqueous solution. Theecaprolactam oligomer was uniformly dissolved in the phosphoric acidaqueous solution. Accordingly, it was impossible to recover thee-caprolactam oligomer from the phosphoric acid aqueous solution.

EXAMPLE 2 5 kg of a molten mixture of 1 part by weight of ecaprolactammonomer, of 4 parts of e-caprolactam oligomer, containing 6.5 percent ofan alkali based on the weight ofthe mixture, were mixed with 1.4 kg ofan aqueous solution containing 55 percent of phosphoric acid. Themixture was further mixed with 10 kg of an aqueous solution of 100Ccontaining 27 percent of so dium dihydrogen phosphate. The mixture wasshaken for 3 minutes to extract the alkali metal ions and phosphoricacid into the phosphate aqueous solution, and

thereafter, settled in a hot separating funnel. About 15 minutes later,the e-caprolactam monomer and oligomer phase was completely separatedfrom the phosphate aqueous solution phase.

Further, the same extraction operation as stated above was successivelyrepeated 9 times using, instead of the fresh phosphate aqueous solution,a mixture solution of 1.4 kg of an aqueous solution containing 57percent of phosphoric acid with the foregoing phosphate aqueous solutionseparated from the e-caprolactam monomer and oligomer phase in theforegoing operation. In each of these repeated opeations 5 kg of thesame molten mixture of the e-caprolactam monomer and oligomer as thatfirst used, was used. The concentrations of the sodium dihydrogenphosphate in the aqueous solution were determined both before and aftereach extraction operation.

Further, the concentrations of sodium dihydrogen phosphate andphosphoric acid in the isolated ecaprolactam monomer and oligomer phasewere deter mined. The results are shown in Table 1.

Table 1 Concentration in purified e-caprolactam monomer and oligomerphase Concentration of NaH PO, in aqueous solution Extrac- From Table 1,it is obvious that the sodium dihydrogen phosphate aqueous solution canbe repeatedly utilized to extract the alkali metal ions and phosphoricacid from the e-caprolactam material phase.

COMPARISON EXAMPLE 2 5 kg of the same mixture of e-caprolactam monomerand oligomer as in Example 2 were mixed into 1 kg of an aqueous solutionof 78 percent of sodium dihydrogen phosphate having a pH of 4.0, and theextraction mixture was shaken for 3 minutes while being maintained at atemperature of 100C or higher. The mixture was settled in a hotseparating funnel. In order to completely separate the e-caprolactammonomer and oligomer phase from the phosphate aqueous solution phase,about 90 minutes were consumed. This is about six times the separationtime of Example 2. The isolated e-caprolactam monomer and oligomer phasecontained 2.35 percent of sodium dihydrogen phosphate.

COMPARISON EXAMPLE 3 5 kg of the same mixture of e-caprolactam monomerand oligomer as used in Example 2 were mixed into 10 kgof an aqueoussolution of 10% of sodium dihydrogen phosphate and the mixture wasshaken for 3 minutes at a temperature of C or higher. The mixture wassettled in a hot separating funnel for 15 minutes. The separatede-caprolactam monomer and oligomer phase contained no alkali metal ion.However, the separated phosphate aqueous solution contained 250 g of e-11 caprolactam monomer which corresponds to one quarter the originalamount of e-caprolactam monomer. This amount of the e-caprolactammonomer in the phosphate aqueous solution was determined by concentrating the aqueous solution to deposit the ecaprolactam monomer fromthe aqueous solution and separating the deposited e-caprolactam monomer.

EXAMPLES 3 THROUGH 7 AND COMPARISON EXAMPLE 4 An e-caprolactam monomeraqueous solution which had been prepared by condensing vapors ofe-caprolactam monomer and water distilled from a system for polymerizinge-caprolactam in the presence of water as a catalyst, was mixed with anaqueous solution of ecaprolactam monomer and oligomer which had beenobtained from washing, with water, poly-e-caprolactam produced in theabove-mentioned polymerization system. The mixture was distilled in thepresence of sodium hydroxide to recover ecaprolactam monomer. Thedistillation residue was collected, and consisted of 25 percent ofe-caprolactam monomer, 2.0 percent of alkali metal ion calculated interms of sodium hydroxide and the balance mainly consisting ofe-caprolactam oligomer.

In Example 3, the above distillation residue was mixed with an aqueoussolution containing sodium dihydrogen phosphate and having a pH of 4.0in an amount of 1.5 times the distillation residue and the mixture wascharged into a settler with a stirrer. The mixture was agitated in thesettler by the stirrer at a temperature of 100C for minutes, and,thereafter, settled for 30 minutes to separate the e-caprolactam monomerand oligomer phase from the phosphate aqueous solution. phase.

The purified e-caprolactam monomer and oligomer phase contained alkalimetal phosphates and phosphoric acid in amounts shown in Table 2. Theecaprolactam monomer and oligomer mixture was mixed with 4 percent ofphosphoric acid based on the weight of the e-caprolactam mixture andthen, sub jected to a batchwise depolymerization process forecaprolactam oligomer. The depolymerization was carried out whileblowing superheated steam into the reacof the original charge was mixedwith the fresh 5- caprolactam monomer and oligomer mixture in an amountfour times the reaction residue. The mixture was subjected to the samedepolymerization and distillation process as stated above. Theseoperations were successively repeated a further three times. That is,the depolymerization were successively carried out 5 times batchwisely.

The e-caprolactam monomer recovered in the five batches were collectedand purified by flowing them through a packed tower-type distillater of6 theoretical stages. The purified e-caprolactam monomer had a PM valueof 3,600 seconds or more and an acid value of 0.002 or less, and wassuitable for producing poly-ecaprolactam usable for fiber making. Theyield of the purified e-caprolactam was 87 percent.

In Example 4, the same procedures as in Example 3 were repeated using anaqueous solution of 40 percent of disodium hydrogen phosphate at a pH of9.5 instead of the sodium dihydrogen phosphate aqueous solution.

In Example 5, the same procedures as in Example 3 were repeated using anaqueous solution containing 40 percent of a mixture of sodium dihydrogenphosphate and disodium hydrogen phosphate in a mole ratio of 1 lat a pHof 7.3.

In Example 6, the same procedures as above were repeated using anaqueous solution containing 40 percent of dipotassium hydrogen phosphateat a pH of 9.6.

In Example 7, the same procedures as above were repeated using anaqueous solution containing 30 percent of lithium dihydrogen phosphateat a pH of 4.0.

In Comparison Example 4, the same procedures as in Example 3 wererepeated except that the e-caprolactam mixture was not treated with thephosphate aqueous solution. The contents of phosphates and phosphoricacid in the e-caprolactam phase isolated from the phosphate aqueoussolution are shown in Table 2. The depolymerizations in Example 4through 7 were all effected without difficulty. However, in ComparisonExample 4, in second and third batches, the depolymerization wasoperationally difficult due to the high viscosity of the chargedmaterial. In fourth batch the reac tion residue could not be subjectedto depolymerization because of its high viscosity.

Table 2 Phosphate aqueous solution Contents of phosphates and phosphoricYield of acid in s-caprolactam mixture /c) purified Example Type ofConcentracaprolactam No. phosphate tion ('7!) MH POf" M HPOf" H -,P

3 NaH P0. 0.15 0.15 87 4 Na HPO 40 0 0.30 O 84 5 NaH PO and 40 0.10 0.100 90 Na HPO.,( 1:1) 6 K HPO 40 0 0.35 0 83 7 LiH PO, 30 0.15 0 0.20 85Comparison Example 4 non Note:

"MH PO ,:monoalkaIi metal dihydrogen phosphate *M,HPO,:dialkali metalhydrogen phosphate '':total yield of batches I, 2 and 3 EXAMPLES 8THROUGH 10 AND COMPARISON EXAMPLES 5 AND 6 In example 8, the sameoperations as in Example 3 were applied to the same e-caprolactammonomer and oligomer mixture as in Example 3 except that the phosphateaqueous solution contained 50 percent of a mix- 13 ture of 7 parts bymole of sodium dihydrogen phosphate and 3 parts by mole of disodiumhydrogen phosphate and had a pH of 5.5, and the extraction was carriedout at a temperature of 170C under a gauge pressure of about 6 kg/cm lnExample 9, the same procedures as in Example 8 were repeated using anaqueous solution of 45 percent of a mixture of 4 parts by mole of sodiumdihydrogen and 6 parts by mole of disodium hydrogen phosphate at a pH of7.4.

In Example 10, the same procedures as in Example 8 were repeated usingan aqueous solution of 40 percent disodium hydrogen phosphate at a pH of9.5.

The contents of phosphates in the isolated ecaprolactam monomer oligomermixture are shown in Table 3. In Examples 8 through 10, thedepolyrnerization was smoothly carried out without difficulty.

Table 3 Content of phosphates in purified e-caprolactam phase InComparison Example 5, the same operations as in Example 8 were repeatedusing an aqueous solution containing 30 percent of sodium chloride at apH of 6.7. The purified s-caprolactam mixture phase contained 0.2percent of sodium chloride and 0.65 percent of sodium ion in the term ofsodium hydroxide. In fourth batch of the depolymerization, the fluidityof the charge lowered undesirably.

In Comparison Example 6, the same procedures as in Example 8 wererepeated using an aqueous solution of a pH of 6.5 containinig 20 percentof sodium sulfate. The purified e-caprolactam phase contained 0.30percent of sodium sulfate and 0.4 percent of sodium ions calculated interms of sodium hydroxide. In this comparison example, the charge in thefourth bath was undesirably highly viscous, and the recoverede-caprolactam monomer smelled like sulphur.

EXAMPLES 11 THROUGH AND omparison EXAMPLES 7 AND 8 In Example 1 l, thesame operations as in Example 3 were applied to a mixture of 80 percentof the same sodium ion-containing e-caprolactam monomer and oligomermixture as in Example 3 and percent of a crude s-caprolactam monomer.The extraction was carried out at a temperature of 80C by using anaqueous solution of a pH of 5.1 containing 20 percent of a mixture of 8parts by mole of sodium dihydrogen phosphate and 2 parts by mole ofdisodium hydrogen phosphate in an amount by weight of 0.7 times theecaprolactam mixture.

Examples 12, 13, 14 and 15 respectively used the phosphate aqueoussolution in concentrations of 30, 40, 60 and 70 percent. The contents ofthe phosphates in the e-caprolactam mixture isolated from the phosphateaqueous solution in Examples 1 1 through 15 are shown in Table 4. Inthese examples, the depolymerization process in every batch was smoothlycarried out without difficulty.

In Comparison Example 7, the same procedures as in Example 1 1 wererepeated using an aqueous solution containing 12 percent of the samephosphate mixture as in Example 11 and having a pH of 5.2. In thiscomparison example, the depolymerization in fifth batch could not becarried out because of very low fluidity of the charge. Further, it wasobserved that in the extraction process, a portion of the s-caprolactammonomer in the e-caprolactam mixture was dissolved out in an amount ofabout 20 percent based on the original amount thereof into the phosphateaqueous solution- In Comparison Example 8, the same operationsas inExample 11 were repeated using an aqueous solution containing percent ofthe same phosphate aqueous solution in Example 1 1 and having a pH of5.1. In this comparison example, the depolymerization in fifth batch wasdifficult because of very low fluidity of the charge. The contents ofthe phosphates in the isolated e-caprolactam mixtures in Examples 1 1through 15 and Comparison Examples 7 and 8 are shown in Table 4.

EXAMPLES 16 THROUGH 18 AND COMPARISON EXAMPLE 9 A waste material from ashaping process of a poly-ecaprolactam which had been made bypolymerizing ecaprolactam in the presence of sodium hydroxide as acatalyst, contained 0.5 percent of sodium ion calculated in terms ofsodium hydroxide. In Example 16, the

waste material was finely divided to an average size of 10 mesh andmixed, at a temperature of 70C, with an aqueous solution of a pH of 7.3containing 55 percent of a mixture of 1 part by mole of sodiumdihydrogen phosphate and 1 part by mole of disodium hydrogen phosphate.The mixture was stirred and settled for 2 hours to extract the sodiumion from the poly-ecaprolactam waste. Therefore, the poly-e-caprolactamwas purified from the phosphate aqueous solution. The concentrations ofthe phosphates in the purified poly-ecaprolactam were determined and,then, the poly-ecaprolactam was burnt in a combustion furnace-Thecombustion was smoothly carried out without difficulty.

In Example 17, the same operations as in Example 16 were repeated exceptthat the waste poly-e-caprolactam was melted and, then, brought intocontact with the same phosphate aqueous solution as in Example 16 at atemperature of 250C.

In Example 18, the same procedures as in Example 16 were repeated exceptthat the waste poly-ecaprolactam was mixed with a crude e-caprolactammonomer in an amount equal to that of the waste polye-caprolactam, andthe mixture was melted, uniformly mixed and, then, brought into contactwith the same phosphate aqueous solution in Example 1 l at a temperatureof C.

In Examples 17 and 18, the combustion of the isolated wastepoly-e-caprolactam was smoothly carried out without blocking of thecombustion bed material.

In Comparison Example 9, the waste poly-e- 16 and thereafter, settledfor 30 minutes to separate the e-caprolactam mixture from the phosphateaqueous solution. The purified e-caprolactam mixture was further mixedwith a fresh crude e-caprolactam monomer the caprolactam was, withoutremoval of the sodium ion same as used above and a fraction of thedistilled etherefrom, burnt in the same combustion furnace. caprolactammonomer which had been distilled in the After the combustionwas'continued for 20 hours, it first distillation step and failed tostand a qualification was observed that the combustion bed material wastest. The mixture was admixed with 1 percent of soblocked. diumhydroxide and subjected to the second batchwise The contents of thephosphates in the isolated 6- 1O distillation.The second distillationwas carried out ata caprolactam mixtures in Examples 16 through 18 andtemperature of 160C at a reflux ratio of 3 to 6 at a dis- ComparisonExample 9 are shown in Table 5. tillation ratio of about 90, using apacked tower type Table 5 distillator having 6 theory stages. Thedistilled ecaprolactam monomer had a PM value of 3,600 seconds or moreand an acid value of 0.002 or less. This Extraction p mixmrep(%) gradeof the e-caprolactam is suitable for producing an Example g Timee-caprolactam polymer usable for fiber making. There- No me C) (hour)NaHzPO NazHPO after, the distillation residue in an amount of about 10is 8 3 8-52 8-83 percent based on the original weight of the e-caprolac-18 190 l 1, 20 tam mixture was mixed with the same fresh crude 6-Comparison 0.50 in the caprolactam monomer and the same unqualifiedfrac- Example 9 2 23:: tion of the distilled e-caprolactam monomer asthose stated above and, then, subjected to the same extraction anddistillation as stated above. Such extraction 25 and distillation weresuccessively repeated four times EXAMPLES 19 THROUGH 23 AND COMPARISONmore.

EXAMPLE 10 In Example 20, the same procedures were repeated In Example19, a mixture of e caprolactam polymer using an aqueous solutioncontaining 40% of disodium and oligomer was depolymerized in thepresence of hydrogen Phosphate and havmg 3 PH of phosphoric acidcatalyst to prepare a crude e-caprolac- In Example e Same preeedures e mExample tam monomer. An aqueous solution of 30 percent of 19 were e e anaqueeuesoluuen e P of the crude e caprolactam monomer was mixed with 17.3 containing 40 percent of a mixture consisting of percent of sodiumpermanganate based on the weight one Part by mole of e m dlhydrogenPhosphate and of the e-caprolactam monomer. The mixture was one part bymole of dlsodium hydrogen phosphate heated at a temperature of about 50Cto deposit man- In Examples 22 and the m Procedures as m ganese dioxide.The crude e-caprolactam solution was Example, were repeated respeeevelyan e' separated from the manganese dioxide by filteration eus m Ofa PHof eemammg 40 percent of and concentrated. The concentrated crudee-caprolac- Potasslum hydrogen P Q P and an aqueous solutam Solution wasmixed with 1 percent of Sodium 40 tion of a pH of 40 containing 30percent of lithium d1- droxide and, then, subjected to the firstdistillation to hydrogen phesphate' recover e-caprolactam. Thedistillation residue con- In cempenson Example the a dlstlnanen P e'tained 7 percent of e-caprolactam oligomer, 8 percent e f as EXamPlewere apphed to the same of amin0capmiC acid, 15 percent of alkali metalion t llation residue as in Example l9 without the extrac- (sodium andpotassium ions) calculated in terms of soe e f alkal' metal diumhydroxide 1 1O of manganese and the The distillation procedures inExamples l9 through ance mainly Consisting of capr0lactam monomen 23were smoothly carried out throughout five batches One part of thedistillation residue obtained above W1theutd'ffeu!ty' was mixed with 3parts of an aqueous solution containa e f this m P the ing percent ofsodium dihydrogen phosphate in a 50 seconddistillation for the first dstillation residue could settler with a stirrer. The mixture wasagitated at a tembe out to dlstlllatlon ratlo of 45 P perature of C for10 minutes to remove alkali but the third distillation could not carryout because of metal ions and manganese from the distillation residue,10W fluldlty of the Charge m the dlsmlater- Table 6 Content ofphosphates and Yield of Phosphate manganese in isolated Average purifiedaqueous solution e-caproluctum mixture distillation e- ExzimpleConcentra- MHZPO4 MZHPO4 Manganese ratio caprolactam No. Phosphate tion(/1) ("/1) ("/1) (p p,m.) (Y/1) l9 NuH PO 50 0.15 H,,Po 3 89 89 20 N-.\HPO 40 0 0.30 l5 89 88 2| NaH PO & 40 0.05 0.09 91 92v Nu- HPO l l 22K2HPO4 40. 0 0.34 I9 88 86 23 Liii Po 30 0.18 HHPO, 4 87 86 Table6-continued Content of phosphates and Yield of*? Phosphate manganese inisolated Average purified aqueous solution e-caprolactam mixturedistillation Example Concentra- MH PO M I-IP0 Manganese ratiocaprolactam No. Phosphate tion (p.p.m.)

Comparison 1 Example 45*! Note:

distillation ratio in the second distillation "a ratio of the totalweight of the purified e-caprolactam obtained in 6 distillations to thetotal weight ofthe crude e-caprolactam used in 6 distillations a totalyield in the first and second distillations EXAMPLES 24 THROUGH 28 ANDCOMPARISON EXAMPLES 11 THROUGH 15 In Example 24, the last distillationresidues in Examples 19 through 23 were incorporated to prepare amixture. The distillation residue mixture was an ecaprolactam mixtureconsisting of 9 percent of sodium ion calculated in terms of sodiumhydroxide, 70 ppm. of manganese, 13 percent by weight of e-caprolactamoligomer, 14 percent of 68-aminocaproic acid and the balance mainlyconsisting of e-caprolactam monomer. The e-caprolactatn mixture wascharged into a settler with a stirrer receiving an aqueous, solutioncontaining percent of a mixture of 8 parts bymole of sodium dihydrogenphosphate and 2 parts by mole of disodium hydrogen phosphate in anamount of five times that of the e-caprolactam mixture, stirred at atemperature of 70C for 20 minutes and, then, settled for 40 minutes toextract the alkali metal ions from the e-caprolactam mixture. Thee-caprolactam mixture phase was separated from the phosphate aqueoussolution.

The purified e-caprolactam mixture was burnt in an experimentarycombustion furnace with a fluidized bed at a temperature ofapproximately 800C.

In examples through 28, the same procedures as in Example 24 wererepeated using the aqueous solutions containing respectively 30, 40, 60and 70 percent of the same phosphate mixture as in Example 24.

In examples 24 through 28, the combustions of the portion of thee-caprolactam monomer in the original distillation residue mixture wasdissolved into the phosphate aqueous solution. As a result of thedissolution, the content of the e-caprolactam monomer in the purifiede-caprolactam mixture was about one quarter the content in the originaldistillation residue mixture. The purified e-caprolactam mixture was notsubjected to the combustion test. y

In the combustion step in Comparison Example 12 the bed material in thefurnace was blocked with dust material hours after starting thecombustion. V

In the combustion step in Comparison Example 13, the bed material wasblocked 8 hours after starting the combustion. Q

In the combustion step of Comparison Example 14, the bed material wasblocked 9 hours after the beginning of the combustion.

In Comparison Example 15, the distillation residue mixture containing 9percent of sodium hydroxide was directly burnt in the same type of thecombustion furnace as used in Example 24. The bed material was blocked 2hours after the beginning of the combustion.

The contents of sodium ioncalculated in terms of phosphate, chloride,sulfate and other compounds and manganese in the isolatede-caprolactammixture are shown in Table 7.

Table 7 Salt aqueous Content of sodium ion and manganese in solutionpurified e-caprolactam mixture Example Concentra- No. Salt tion NaH PONa HPO NaCl Na SO NaOH Manganese (ppm) Comparison Phosphate Example llmixture 12 1.3 0.60 I0 24 20 0.25 0.08 5 25 30 0.17 0.04 3 26 40 0.160.04 3 27 0.20 0.04 2 2s 0.30 0.03 4 Coml2 2.3 0.30 l8 pari- 13 NaCl0.16 3.0 30

son Exam- [4 NaSO 20 0.26 2.3 30 ple l5 9.0 70

isolated e-caprolactam mixture were smoothly carried out withoutblocking of the fluidized bed with dust ma- 60 EXAMPLE 29 and compansonExample 16 terial. Three parts of a crude e-caprolactam which had beenIn Comparison Examples 1 1 through 14, the same distilled together withwater from a system for polymeroperations as in Example 24 were repeatedusing, inizing e-caprolactam in the presence of water as a catasteadofthe phosphate aqueous solutions used in Examlyst, was mixed with onepart of a mixture of eples 24 through 28, aqueous solutions respectivelycontaining 12 percent (Comparison Example 11) and 80 percent (ComparisonExample 12) of thesame phosphate mixture as used in Example 24, 30percent of socaprolactam monomer and oligomer which had been obtained bywashing, with water, the poly-e-caprolactam prepared in the abovepolymerization system. The mixture thus prepared was further admixedwith 0.6

percent of sodium hydroxide and, then, subjected to a distillation torecover purified e-caprolactam monomer. The distillation residue was ane-caprolactam mixture consisting of 30 percent of e-caprolactamoligomer, 4.0 percent sodium ion calculated in terms of sodium hydroxideand the balance mainly consisting of e-caprolactam monomer. One part ofthe distillation residue was charged into a settler with an agitatorreceiving one part of an aqueous solution containing 50 percent of amixture of 4 parts by mole of sodium dihydrogen phosphate and 6 parts ofdisodium hydrogen phosphate. The mixture in the settler was stirred at atemperature of 140C under a gauge pressure of about 3 kg/cm for minutesand, thereafter, settled for 30 minutes to separate the e-caprolactammixture from the phosphate aqueous solution.

The purified e-caprolactam mixture was admixed with wastepoly-e-caprolactam in an amount 3 times the isolated e-caprolactammixture. The mixture was subjected to batchwise depolymerizationsrepeated five times. In the first depolymerization, the content ofphosphoric acid as a depolymerization catalyst was about 6 percent. Eachdepolymerization was effected at a temperature of 250C while supplyingsuperheated steam into the depolymerization system under a normalpressure with a depolymerization ratio of about 80 percent. The crudee-caprolactam monomer thus obtained was fed into a packed tower typedistillater having 6 theoretical stages from which a purifiede-caprolactam monomer was obtained. The reaction residue in an amount ofabout percent based on the original amount of the e-caprolactam materialmixture fed into the depolymerization, was mixed with the same wastepoly-e-caprolactam used in the above in the same manner as above. Themixture was subjected to the same depolymerization and distillation asdetailed above. These procedures were repeated three times more withoutdifficulty. The distilled e-caprolactam monomer had a PM value of 3,600seconds or more and an acid value of 0.002 or less and was suitable forproducing poly-e-caprolactam usable for fiber makingg. As a result ofthe repeated depolymerizations and distillations, the purifiede-caprolactam monomer was obtained in a yield of 88 percent.

In Comparison Example 16, the same distillation residue containing 4.0percent of sodium hydroxide as used in Example 29 was directly mixedwith the same waste poly-e-caprolactam as used in Example 29. Themixture was subjected to the same depolymerizations and distillations asin Example 29. In third and fourth batches of the depolymerization, thefluidity of the charge was undesirably decreased. In fifth batchdepolymerization was impossible because of very low fluidity of thecharge. The total yield of the purified ecaprolactam monomer in firstthrough fourth batches was 73 percent.

The contents of the phosphates in the isolated ecaprolactam materialmixture in Example 8 are shown in Table 8.

Table 8 Content of sodium ion in Yield of e-caprolactam mixture (71)purified Note: Total yield in batches 1 through 5. "Total yield inbatches 1 through 4.

EXAMPLES 30 and 31 and Comparison Example 17 In Example 30, a mixture of700 parts of waste polye-caprolactam and 300 parts of e-caprolactamoligomer containing 0.5 percent of sodium ion calculated in terms ofsodium hydroxide, was continuously depolymerized in the presence ofphosphoric acid catalyst to recover the resultant e-caprolactam monomer.50 parts of reaction residue were obtained. The reaction residue was ane-caprolactam material mixture consisting 7 parts of phosphoric acid, 6parts of sodium dihydrogen phosphate, 3 parts of titanium dioxidepigment, 510 ppm of manganese of the balance mainly consisted ofe-caprolactam monomer, oligomer and polymer and a brown resinoussubstance which was a by-product of the depolymerization. One part ofthis reaction residue was mixed with 2 parts of aqueous solutioncontaining 50 percent of disodium hydrogen phosphate and charged into asettler with a stirrer to remove phosphoric acid and sodium ions andmanganese. The

mixture was stirred at a temperature of 200C under a gauge pressure ofabout 1.4 kg/cm for 10 minutes and, then, settled for 30 minutes.Thereafter, the purified e-caprolactam material mixture was burnt in acombustion furnace having a fluidized bed consisting of sand.

In Example 31, the same operations as in Example 30 were repeated usingan aqueous solution containing 40 percent of trisodium phosphate in anamount three times the amount of the distillation residue.

The purified e-caprolactam material mixtures in Examples 30 and 31contained ash, phosphates, phosphoric acid and manganese in amounts asshown in Table 9, and smoothly burnt without blocking the combustionbed. The combustion residue ashes collected into a cyclone containednon-burnt combustible substance in amounts shown in Table 9.

In Comparison Example 17, the same reaction residue as in Example 30 wasdirectly burnt in the same manner as in Example 30. 2 hours later, thebed mate rial consisting of sand was sintered. The combustion residueashes contained 80% of non-burnt combustible substance in the form ofdust.

Examples 30 and 31 and Comparison Example 17 are summarized in Table 9.

Table 9 Example 30 Comparison Example 1 7 Example 3 1 Extract- Na PO ionNa H P0 Content In purified caprolactam mixture NaHPO,

Manganese 2 blocked 2 none Combustion EXAMPLES 32 through 34 andComparison Example 18 solution. The purified e-caprolactam materialmixture was mixed with 500 parts of the same fresh e-caprolactammaterial mixture as used first, and phosphoric acid in an initialconcentration of 3 percent and the mixture In Example 32, 1000 parts ofan e-caprolactam mate- 5 WaS smoothly depolymerized without difficultywhile rial mixture consisting of 20 percent of an e-caprolacdistillingout the resultant e-caprolactam monomer. tam monomer, 2.0 percent ofsodium ion calculated in The depolymerization was effected at atemperature of terms of sodium hydroxide and the balance mainly con-260C by feeding superheated steam into the depolymsisting of ane-caprolactam oligomer was depolymererization mixture. ized batchwise inthe presence of phosphoric acid cata- By the three depolymerizationsstated above, the lyst, while distillating out the resultante-caprolactam cru e-capr lactam monom r wa r c r in a y l monomer. 250parts of reaction residue were obtained. of 95 per n Th Crude p o ammonomer a The on re ue Contained 32 parts of phosphoric purified into apure grade suitable for fiber making in a acid and 60 parts of sodiumdihydrogen phosphate. A yi O 92 p r n by y Of'distillatiol'llarge amountof the phosphate was generally not dis- 1h Example the Same Procedures35 in Example solved in the depolymerization i t d hi 32 were repeatedusing an aqueous solution containing sulted in an undesirable decreasein fluidity of the de- 35 pe nt of sodium dihydrogen phosphate in aweight polymerization mixture. of four times the e-caprolactam mixture.

250 parts of the reaction residue were mixed with In Ex mp the Sameoperations as in mp 32 750 parts of the same fresh e-caprolactam ma i lwere repeated using an aqueous solution containing mixture as usedabove. After the content of free phospe ent of disodium hydrogenphosphate in a weight phoric acid in the mixture was adjusted to 31parts, the equal to that of the p l m m r mixture was subjected to thedepolymerization in the In Comparison Example the Same operations samemanner as stated above. When the reaction resi- 5 in Example 32 Wererepeated except that the Second due reached the amount of 450 parts, thefluidit f the 2 depolymerization and distillation were carried out untildepolymerization mixture, that is, the reaction residue h reacti nresidue became an amount of 350 parts in became insufficient. Thereaction residue contained 31 spite of a remarkable ecreas in thefluidity of the reparts of phosphoric acid and 105 parts of sodiumdihyaction residue and the fact that the extraction process drogenphosphate. This reaction residue was dis- 0 for the e-caprolactammixture was omitted. In the comcharged from the depolymerization vesseland mixed 3 paris n xamp the crude eap la ta m n m r with 200 parts ofadistillation residue consisting of 10 was obtained in a yield of 87percent. Ho the percent of an e-caprolactam oligomer and 90 percentcrude e-caprolactam thus obtained noticeably colored of an e-caprolactammonomer which had been obbrown. Further, the upper portion of the insidewall tained from a distillation process ofa crude e-caprolacanddistillation exit of the depolymerizing vessel were tam. Thee-caprolactam material mixture thus prepared 35 Stained With a arg ou tof an a es e b own Subwas mixed with an aqueous solution containing 35perstance having a low fluidity. cent of a mixture of sodium dihydrogenphosphate and In Examples 32 through 34, the purified e-caprolacdisodiumhydrogen phosphate in the same amount by tam monomer had a PM value of3,600 seconds, or mole and charged into a settler with a stirrer, toremove more and an acid value of 0.002 or less, and was pertiphosphoricacid and phosphates from the e-caprolacnent for producingpoly-e-caprolactam usable for fiber tam material mixture. The ratio byweight of the 6- making. I caprolactam material mixture to the phosphatesolu- The results of Examples 32 through 34 and Comparitlon was 1 1 Themixture was Stlrfed at a mp son Example 18 are summarized in Table 10.From this lure of 106 C under a hormal Prassur6 for 20 table, it isobvious that the yields of the crude and puri- Settled for 40 minutes,thereafter, the 45 fied e-caprolactam in Examples 32 through 34 arecaprolactam mixture was separated from the phosphate hi h h those iComparison Example 1 Table 10 Example Example Example Comparison 32 3334 Example 18 Extraction Salt NaH PO and NaH Pog Na HFO Na i-1P0, Ratioof el/l.5 1 H4 H1 caprolactam mixture weight to phosphate solutionweight Content in H PO 0 0.18 0 purified e- NaH PO, 0.12 0.15 0.08caprolactam mixture /e) Na l-llo 0.04 0 0.l2 Yield of crude 95 94 92 87e-caprolactam Yield of purified 92 91 88 82 ecaprolactam (7:-)

In Example 35, 1,000 parts of waste poly-e-caprolactam was depolymerizedin the presence of phosphoric acid catalyst while distilling out theresultant ecaprolactam monomer. 60 parts of reaction residue werecollected, wherein 15 parts of phosphoric acid and 800 ppm of manganesewere contained and a brown resinous substance deposited. The reactionresidue was mixed with an aqueous solution of a pH of 8.0 containing 15percent ofa mixture of 3 parts by mole of disodium hydrogen phosphateand 2 parts by mole of sodium dihydrogen phosphate in an amount of seventimes the weight of the reaction residue. The mixture was stirred at atemperature of 190C for 15 minutes to extract phosphoric acid andphosphates from the reaction residue to the phosphate aqueous solution,and the ecaprolactam material mixture was separated from the phosphateaqueous solution using a centrifugal separator at a centrifugal effectof 150 G for minutes. The purified e-caprolactam material mixture wasburnt in a combustion furnace without blocking of the bed material.

In Examples 36 through 41, the same procedures as in Example 35 wererepeated using aqueous solutions respectively containing 20, 30, 40, 50,60 and 70 percent of the same phosphate mixture as in Example 35. Inevery example, the combustion of the isolated ecaprolactam materialmixture was smoothly carried out without blocking of the bed material.

In Comparison Examples 19 and 20, the same procedures as in Example 35were repeated using aqueous solutions respectively containing and 80percent of the same phosphate mixture as used in Example 35. InComparison Example 19, the bed material composed of sand in thecombustion furnace was blocked 10 hours after the beginning ofthe'combustion. In Com- EXAMPLES 42 THROUGH 44 AND COMPARISON EXAMPLE 21In Example 42, 3,000 parts of waste e-caprolactam polymer werecontinuously depolymerized in the presence of phosphoric acid catalyst,and 100 parts of reaction residue were collected. This reaction residuecontained 1 part of phosphoric acid and a small amount of brown resinoussubstance. In order to remove phosphoric acid and phosphates, thereaction residue in an amount of 3 parts was mixed with 2 parts of anaqueous solution of a pH of 7.3 containing 30 percent of a mixture oflithium dihydrogen phosphate and dilithium hy drogen phosphate in thesame amount by mole. The mixture was stirred at a temperature of 230Cfor 10 minutes in a settler with a stirrer and then settled for minutesand, thereafter, the e-caprolactam material mixture was separated fromthe phosphate aqueous solution. The purified e-caprolactam materialmixture was burnt in a combustion furnace with a fluidized bed. Thecombustion was smoothly carried out over 48 hours without blocking ofthe bed.

In Examples 43 and 44, the same operations as in Example 42 were carriedout using aqueous solutions respectively containing 40 percent ofamixture of sodium dihydrogen phosphate and disodium hydrogen phosphatein the same amount by mole (Example 43) and 40 percent of a mixture ofpotassium dihydrogen phosphate and dipotassium hydrogen phosphate in thesame amount by mole (Example 44).

In these examples, the combustions were smoothly effected over 48 hourswithout difficulty.

In Comparison Example 21, the same reaction residue as in Example 42 wasdirectly burnt in the same type of the combustion furnace as used inExample 42. 10 hours later, the bed composed of sand was blocked.

The contents of impurities in the e-caprolactam mixture subjected tocombustion are shown in Table 12.

mixture ('70) parison Example 20, the bed material composed of 50EXAMPLE 45 AND COMPARISON EXAMPLE 22 sand was blocked 20 hours after thebeginning of the combustion.

The contents of impurities in the isolated e-caprolactam materialmixtures in these examples and comparison examples are shown in Table 11.

Table l 1 Content of impurities in purified e-caprolactam ide and 1 partof potassium hydroxide while distilling the resultant e-caprolactammonomer. The reaction residue which has been collected from the aboveprocess, was an e-caprolactam material mixture containing 250 ppm ofmanganese, 60 ppm of copper and 25 percent of alkali metal ions in theterm of sodium hydroxide.

In order to eliminate metal ions, the reaction-residue was mixed with anaqueous solution ofa pH of 7.4 containing 50 percent of a mixture ofpotassium dihydrogen phosphate and disodium hydrogen phosphate in thesame quantities in an amount of five times the weight of the reactionresidue, and the mixture was charged into a settler with a stirrer. Themixture was stirred at a temperature of 230C under a gauge pressure of24 kg/cm for minutes and, settled for minutes and, thereafter, thee-caprolactam material mixture was separated from the phosphate aqueoussolution. The purified e-caprolactam material mixture containedimpurities in the contents as shown in Table 13. The e-caprolactammaterial mixture thus isolated was burnt at a temperature of about 800Cin an experimental combustion furnace having a fluidized bed. Thiscombustion was smoothly continued over 48 hours without difficulty.

In Comparison Example 22, the same reaction residue as used in Example45 was directly burnt in the same manner as in Example 45, without theremoval of the metal ions.

In the combustion, one hour after beginning of the combustion, the bedbegan blocking and 2 hours after the start, it was impossible tocontinue the combustion due to the firm blocking of the bed.

Table 13 A mixture of e-caprolactam monomer and oligomer was extractedfrom a polymerized e-caprolactam with water. The mixture was distilledin the presence of sodium hydroxide to recover the e-caprolactammonomer. The distillation residue collected from the above distillationwas an e-caprolactam material mixture consisting of 1.0 percent ofsodium ion calculated in terms of. sodium hydroxide, about 20 percent ofan ecaprolactam monomer and the balance mainly consisting of ane-caprolactam oligomer. This residue which is referred to hereinafter assingle distillation residue, was mixed with an aqueous solution of 45percent of a mixture of 2 parts by mole of disodium hydrogen phos phateand 3 parts by mole of sodium dihydrogen phosphate in an amount the sameas the weight of the distillation residue. The mixture was charged intoa settler with a stirrer, stirred at a temperature of 180C for 15minutes, and settled for 30 minutes, and the ecaprolactam materialmixture was separated from the phosphate aqueous solution. Thisextraction is referred to hereinafter as first extraction.

The isolated e-caprolactam material mixture was continuouslydepolymerized in the presence of -percent of phosphoric acid catalyst ata temperature of 240C under a pressure of 0.5 absolute atmospheres whiledistilling out the resultant e-caprolactam monomer by blowingsuperheated steam into the e-eaprolactam mixture. As a result of theseoperations, parts ofreaction residue was obtained from 1,700 parts ofthe isolated e-caprolactam material mixture.

In order to purify the resultant crude e-caprolactam monomer, 0.2percent of sodium hydroxide was mixed with the crude e-caprolactammonomer and the mixture was distilled at a temperature of C using apacked tower type distillator having six theoretical stages. About 3percent of the distillation residue based on the original weight of thecrude e-caprolactam monomer were collected. The reaction residue in thedepolymerization process was admixed with the distillation residue inthe refining distillation process. This mixture is referred to asprocess residue admixture. The major parts of the phosphate aqueoussolution recovered from the first extraction process was reused toextract the phosphoric acid and alkali metal ions from the processresidue admixture. This extraction is referred to hereinafter as secondextraction." The second extraction was carried out in the similar mannerto the first extraction, and an e-caprolactam material mixture wasseparated from the phosphate aqueous solution and, thereafter, burntwithout difficulty.

The same operations as detailed above were repeated nine times more.Throughout these times, a phosphate aqueous solution was repeatedly usedfor the first and second extractions in the same manner asv statedabove. In the start of each of these times, the concentration of thephosphate aqueous solution is regulated by adding a necessary amount ofphosphoric acid or water.

By these ten treatments, the crude e-caprolactam monomer was obtained ina yield of 95 percent based on the total weight of the singledistillation residues and the purified e-caprolactam monomer in a yieldof 89 percent.

The first extracted e-caprolactam material mixtures contained an averageof 0.10 percent of sodium dihydrogen phosphate and 0.08 percent ofdisodium hydrogen phosphate. Also, the second extracted ecaprolactammaterial mixture contained an average of 0.14 percent of sodiumdihydrogen phosphate and 0.05 percent of disodium hydrogen phosphate.

Through these ten treatments, 41 parts of phosphoric acid with respectto 17,000 parts of the single distillation residue were consumed for thefirst and second extractions. Also, 400 parts of phosphate mixtureconsisting of disodium hydrogen phosphate and sodium dihy' drogenphosphate in an average ratio by mole of 7 l, were recovered from theten treatments.

Compared with this, provided the first and second extractions are eachcarried out using a fresh phosphate aqueous solution which is the sameas in the present invention, 243 parts of phosphoric acid and 143 partsof sodium hydroxide with respect to 17,000 parts of the singledistillation residue will be consumed.

EXAMPLES 47 THROUGH 54 A poly-e-caprolactamwas depolymerized in thepresence of phosphoric acid while distillingout the resultante-caprolactam monomer. The reaction residue was an e-caprolactammaterial mixture containing 5 percent of phosphoric acid and anoticeable amount of a brown resinous substance. The distillationresidue was divided into eight fractions.

In Example 47, one fraction of the reaction residue was mixed with anaqueous solution of a pH of 4.0 containing 40 percent of sodiumdihydrogen phosphate in amount of 2.5 times the weight of the fraction.The mixture was charged into a settler with stirrer, stirred at atemperature of 200C for 20 minutes, and settled for 40 minutes and,thereafter, the e-caprolactam material mixture phase was separated fromthe phosphate aqueous solution.

In Examples 48 through 54, the same procedures as in Example 47 wererepeated respectively using phosphate aqueous solutions of a pH as shownin Table 14. The contents of impurities in the isolated e-caprolactammixtures in Examples 47 through 54 are shown in Table l4.

4. A method as claimed in claim 1, wherein said 6- caprolactam materialis selected from the group consisting of:

a. concentrated products or distillation residues of aqueous solutionscontaining an e-caprolactam monomer or oligomer or a mixtures of themonomer and the oligomer produced by washing a ecaprolactam polymer withwater,

b. distillation residues of aqueous solutions containing ane-caprolactam monomer, obtained from processes for polymerizing ane-caprolactam monomer in the presence of water as a catalyst, wherein aportion of the e-caprolactam monomer is evaporated together with thewater,

c. reaction residues produced in processes for depolymerizing ane-caprolactam polymer or oligomer Table 14 Content of impurities inPhosphate purified e-caprolactam solution mixture ("/r) Example No.Composition pH H PO, NaH PO Na HPO Na PO 47 NaH- PO 4.0 0.25 0.20 0.000.00

NaH PoJN-a HPo 48 (9:1 by mole) 4.6 0.15 0.14 0.00 0.00

NaH PO /Na- HPO 49 (4:6 by mole) 7.2 0.00 0.1 l 0.07 0.00 50 Na HPO 8.20.00 0.04 0.15 0.00

Na HPO /Na;,PO 51 (9:1 by mole) 9.6 0.00 0.03 0.25 0.00

Na HPO INa PO 52 (8:2 by mole) 10.2 0.00 0.00 0.25 0.05

Na HPO /Na ,PO, 53 [4:6 by mole) 11. 0.00 0.00 0.17 0.13 54 NzmPO, 12.10.00 0.00 0.06 0.30

Table 14 shows that the total content of the phos phoric acid andphosphates is relatively small in Examples 49 and 50 wherein the pH ofthe phosphate aqueous solutions are in a range of from 5 to 9. However,all of the purified e-caprolactam mixtures in Examples 47 through 54were smoothly burnt over 48 hours without difficulty.

What we claim is: 1. A method for purifying an e-caprolactam materialcomprising the steps of:

bringing an e-caprolactam material selected from the group consisting ofa monomer, oligomers, and polymers of e-caprolactam and mixtures of twoor more of the above-mentioned compounds and containing at least oneimpurity compound selected from the group consisting of phosphoric acidand ionized metal compounds, at a temperature of at least 60C, intocontact with an aqueous solution containing to 75 percent by weight ofat least one alkali metal phosphate to extract said impurity compoundfrom said e-caprolactam material into said alkali metal phosphateaqueous solution, and

separating said e-eaprolactam material phase from said alkali metalphosphate aqueous solution phase.

2. A method as claimed in claim 1, wherein said alkali metal phosphateaqueous solution is in a concentration of to 60 percent by weight.

3. A method as claimed in claim 1, wherein said alkali metal phosphateis selected from the group consisting of monoalkali metal dihydrogenphosphates, dialkali metal monohydrogen phosphates, trialkali metalphosphates and mixtures of two or more of the abovementioned phosphates.

or a mixture of the polymer and oligomer in the presence of phosphoricacid as a catalyst, d. distillation residues produced in processes forrefining a crude e-caprolactam monomer which has been produced bydepolymerizing an e-caprolactam polymer or oligomer or a mixture of thepolymer and oligomer in the presence of phosphoric acid as a catalyst,distillation residues produced in processes wherein a crudee-caprolactam monomer is produced by the Beckman rearrangement reactionof cyclohexanone oxime and the resultant crude e-caprolactam isdistilled in the presence of an alkaline alkali metal compound reactionresidues produced in processes for depolymerizing an e-caprolactampolymer or oligomer or a mixture of the polymer and oligomer in thepresence of an alkali metal compound as catalyst and g. mixtures of twoor more of the e-caprolactam materials as stated above.

5. A method as claimed in claim 1, wherein said contact is effected at atemperature of 60 to 300C.

6. A method as claimed in claim 5, wherein said contact temperature isfrom to 250C.

7. A method as claimed in claim 1, wherein a ratio by weight of saide-caprolactam material to said alkali metal phosphate aqueous solutionis l l0 to 2 l.

8. A method as claimed in claim 1, wherein said alkali metal phosphateaqueous solution is in a pH of 4 to 12.5.

9. A method as claimed in claim 8, wherein said pH of said alkali metalphosphate aqueous solution is from to 9.

10. A method as claimed in claim 1, wherein an ecaprolactam materialcontaining alkali metal ions and another e-caprolactam materialcontaining phosphoric acid are simultaneously brought into contact witha common said alkali metal phosphate aqueous solution.

11. A method as claimed in claim 1, wherein an ecaprolactam materialcontaining alkali metal ions and an impure e-caprolactam materialcontaining phosphoric acid are separately brought into contact with acommon said alkali metal phosphate aqueous solution.

12. A method as claimed in claim 1, wherein said purified e-caprolaetammaterial is destroyed in a fluidized bed combustion furnace.

13. A method as claimed in claim 1, wherein said purified e-caprolactampolymer or oligomer is subjected to depolymerization to recover thee-caprolactam monomer.

14. A method as claimed in claim 1, wherein said purified e-caprolactamis further purified by way of distillation 15. A method as claimed inclaim 1, wherein said ionized metal compounds are ionized alkali metalcompounds.

17. A method as claimedin claim 1, wherein said separating is carriedout by centrifuging the mixture of said e-caprolactam material and saidalkali metal phosphate aqueous solution.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION 6 Patent No.3,925,365 Dated December 9, 1975 lnventofls) Nobuo Izawa and ToshihikoKohno It is certified that error appears in the above-identified patent6 and that said Letters Patent are hereby corrected as shown below:

In Table 7, on the eighth line in the next to the last column, delete"30" and insert therefor 3.'O

In Table 7, on the eighth line in the last column,

insert 30 Q In Table 9, in the last column, after "blocked 2" inserthrs. later-- Signed and Sealed this I sixteenth Day of March 1976 [SEAL]Attest:

0 RUTH c. MASON c. MARSHALL DANN Arie-fling Offic Commissioner ofPatemsand Trademarks

1. A METHOD FOR PURIFYING AN $-CAPROLACTAM MATERIAL COMPRISING THE STEPS OF: BRINGING AN $-CAPROLACTAM MATERIAL SELECTED FROM THE GROUP CONSISTING OF A MONOMER, OLIGOMERS, AND POLYMERS OF $CAPROLACTAM AND MIXTURES OF TWO OR ,ORE OF THE ABOVEMENTIONED COMPOUNDS AND CONTAINING AT LEAST ONE IMPURITY COMPOUND SELECTED FROM THE GROUP CONSISTING OF PHOSPHORIC ACID AND IONIZED METL COMPOUNDS, AT A TEMPERATURE OF AT LEAST 60*C, INTO CONTACT WITH AN AQUEOUS SOLUTION CONTAINING 15 TO 75 PERCENT BY WEIGHT OF AT LEAST ONE ALKALI METAL PHOSPHATE TO EXTRACT SAID IMPURITY COMPOUND FROM SAID $-CAPROLACTAM MATERIAL INTO SAID ALKALI METAL PHOSPHATE AQUEOUS SOLUTION, AND SEPARATING SAID $-CAPROLACTAM MATERIAL PHASE FROM SAID ALKALI METAL PHOSPHATE AQUEOUS SOLUTION PHASE.
 2. A method as claimed in claim 1, wherein said alkali metal phosphate aqueous solution is in a concentration of 30 to 60 percent by weight.
 3. A method as claimed in claim 1, wherein said alkali metal phosphate is selected from the group consisting of monoalkali metal dihydrogen phosphates, dialkali metal monohydrogen phosphates, trialkali metal phosphates and mixtures of two or more of the above-mentioned phosphates.
 4. A method as claimed in claim 1, wherein said epsilon -caprolactam material is selected from the group consisting of: a. concentrated products or distillation residues of aqueous solutions containing an epsilon -caprolactam monomer or oligomer or a mixtures of the monomer and the oligomer produced by washing a epsilon -caprolactam polymer with water, b. distillation residues of aqueous solutions containing an epsilon -caprolactam monomer, obtained from processes for polymerizing an epsilon -caprolactam monomer in the presence of water as a catalyst, wherein a portion of the epsilon -caprolactam monomer is evaporated together with the water, c. reaction residues produced in processes for depolymerizing an epsilon -caprolactam polymer or oligomer or a mixture of the polymer and oligomer in the presence of phosphoric acid as a catalyst, d. distillation residues produced in processes for refining a crude epsilon -caprolactam monomer which has been produced by depolymerizing an epsilon -caprolactam polymer or oligomer or a mixture of the polymer and oligomer in the presence of phosphoric acId as a catalyst, e. distillation residues produced in processes wherein a crude epsilon -caprolactam monomer is produced by the Beckman rearrangement reaction of cyclohexanone oxime and the resultant crude epsilon -caprolactam is distilled in the presence of an alkaline alkali metal compound f. reaction residues produced in processes for depolymerizing an epsilon -caprolactam polymer or oligomer or a mixture of the polymer and oligomer in the presence of an alkali metal compound as catalyst and g. mixtures of two or more of the epsilon -caprolactam materials as stated above.
 5. A method as claimed in claim 1, wherein said contact is effected at a temperature of 60* to 300*C.
 6. A method as claimed in claim 5, wherein said contact temperature is from 100* to 250*C.
 7. A method as claimed in claim 1, wherein a ratio by weight of said epsilon -caprolactam material to said alkali metal phosphate aqueous solution is 1 : 10 to 2 :
 1. 8. A method as claimed in claim 1, wherein said alkali metal phosphate aqueous solution is in a pH of 4 to 12.5.
 9. A method as claimed in claim 8, wherein said pH of said alkali metal phosphate aqueous solution is from 5 to
 9. 10. A method as claimed in claim 1, wherein an epsilon -caprolactam material containing alkali metal ions and another epsilon -caprolactam material containing phosphoric acid are simultaneously brought into contact with a common said alkali metal phosphate aqueous solution.
 11. A method as claimed in claim 1, wherein an epsilon -caprolactam material containing alkali metal ions and an impure epsilon -caprolactam material containing phosphoric acid are separately brought into contact with a common said alkali metal phosphate aqueous solution.
 12. A method as claimed in claim 1, wherein said purified epsilon -caprolactam material is destroyed in a fluidized bed combustion furnace.
 13. A method as claimed in claim 1, wherein said purified epsilon -caprolactam polymer or oligomer is subjected to depolymerization to recover the epsilon -caprolactam monomer.
 14. A method as claimed in claim 1, wherein said purified epsilon -caprolactam is further purified by way of distillation.
 15. A method as claimed in claim 1, wherein said ionized metal compounds are ionized alkali metal compounds.
 16. A method as claimed in claim 1, wherein said separating is carried out by settling the mixture of said epsilon -caprolactam material and said alkali metal phosphate aqueous solution.
 17. A method as claimed in claim 1, wherein said separating is carried out by centrifuging the mixture of said epsilon -caprolactam material and said alkali metal phosphate aqueous solution. 